Oxy-Fuel Furnace and Method of Heating Material in an Oxy-Fuel Furnace

ABSTRACT

An oxy-fuel furnace and method of heating material in an oxy-fuel furnace are disclosed. The method includes combusting oxygen and fuel with an oxy-fuel burner arrangement in the oxy-fuel furnace forming combustion gases, and maintaining a vortex including the combustion gases within a central region of an enclosure of the oxy-fuel furnace. The oxy-fuel burner arrangement includes a plurality of high momentum oxy-fuel burners arranged at an angle to generate the vortex, the angle being greater than 15 degrees but less than 75 degrees with respect to a furnace wall boundary of the enclosure, an angular velocity of greater than 0.07 radians per second, or a combination thereof. The furnace includes an oxy-fuel burner arrangement including at least two high momentum oxy-fuel burners having high shape factor nozzle geometries, and an enclosure. The vortex increases convective heating within the enclosure and uniformity of heating within the enclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and benefit of U.S. ProvisionalPatent Application No. 61/471,900, filed Apr. 5, 2011, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to systems and methods of heatingmaterials. More specifically, the present invention is directed tooxy-fuel furnaces and methods of heating material by using oxy-fuelfurnaces.

Nitrogen oxides (NOx) are among the primary air pollutants emitted bycombustion processes. Because NOx promotes the formation of harmfulatmospheric reaction products that cause smog, air quality standardshave been imposed by various government agencies to limit the amount ofNOx that can be emitted into the atmosphere. As a result of theincreasing environmental legislation in many countries and increasingglobal awareness of atmospheric pollution, modern combustion technologyhas been improved to reduce NOx emissions from many types of combustionequipment.

The secondary metals industry is generally considered to be a majorsource of NOx pollution and therefore is subject to stringentregulations on NOx emissions. The reduction of NOx production incombustion processes becomes more important in this industry as thedemand for metals increases while environmental regulations on NOxbecome increasingly stringent. Full oxy-fuel combustion theoreticallycan produce very low NOx emissions due to the lack of nitrogen in theoxidant.

The secondary metals industry has had innovation that reduces NOxemissions. Such a known system is described in U.S. Pat. App. Pub. No.2007/0254251, which is hereby incorporated by reference in its entirety.The known system achieves spacious combustion by entraining furnacegases into a flame zone. Such a system reduces NOx emissions. However,further reductions are desirable, especially if the further combustionNOx reductions are balanced with heat energy consumption concerns, forexample, by balancing radiative and convective heat transfer components.

Traditional low-momentum oxy-fuel combustion is dominated by radiativemode heat transfer but lacks a convective component of heating. The lackof convective component is due to the low gas volumes and can increasethe potential for inconsistent or uneven heating, hot spots, and thegeneration of NOx. In contrast, air fuel combustion lacks efficiency ofradiative heating because of N₂-dilution. However, air fuel combustioncan have a strong convective heat transfer component because of higherflue gas volumes that can be useful in heating a product when combinedwith radiation. However, the radiation from an air-fuel flame is muchlower than the radiation from an oxy-fuel flame.

An oxy-fuel furnace and method of heating material in an oxy-fuelfurnace that do not suffer from one or more of the above drawbacks wouldbe desirable in the art.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment, a method for heating material in an oxy-fuelfurnace includes combusting oxygen and fuel with an oxy-fuel burnerarrangement in the oxy-fuel furnace forming combustion gases, andmaintaining a vortex including the combustion gases within a centralregion of an enclosure of the oxy-fuel furnace. The oxy-fuel burnerarrangement includes a plurality of high momentum oxy-fuel burnersarranged at an angle to generate the vortex, the angle being greaterthan 15 degrees but less than 75 degrees with respect to a furnace wallboundary of the enclosure.

In another exemplary embodiment, a method for heating material in anoxy-fuel furnace includes combusting oxygen and fuel with an oxy-fuelburner arrangement in the oxy-fuel furnace forming combustion gases, andmaintaining a vortex including the combustion gases within a centralregion of an enclosure of the oxy-fuel furnace. The vortex has anangular velocity of greater than 0.07 radians per second.

In another exemplary embodiment, an oxy-fuel furnace includes anoxy-fuel burner arrangement including at least two high momentumoxy-fuel burners having high shape factor nozzles, and an enclosure. Theoxy-fuel burner arrangement includes a plurality of high momentumoxy-fuel burners arranged at an angle to generate a vortex, the anglebeing greater than 15 degrees but less than 75 degrees with respect to afurnace wall boundary of the enclosure. The vortex increases convectiveheating within the enclosure and uniformity of heating within theenclosure.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 through 4 are schematic drawings of oxy-fuel furnaces accordingto embodiments of the disclosure.

FIG. 5 is a three-dimensional schematic drawing of an oxy-fuel furnaceaccording to an embodiment of the disclosure.

FIG. 6 is a graphical comparison of an exemplary method of heatingmaterial in an oxy-fuel furnace according to the disclosure and othermethods of heating material.

FIG. 7 is a graphical illustration of surface temperature as a functionof time for an exemplary oxy-fuel furnace according to the disclosure.

FIG. 8 is a comparative graphical illustration of surface temperaturesof furnace walls and material due to operation of an oxy-fuel furnacewithout forming a vortex.

FIG. 9 is a graphical illustration of surface temperatures of furnacewalls and material due to operation of an oxy-fuel furnace forming avortex according to the disclosure.

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION OF THE INVENTION

Provided is an exemplary oxy-fuel furnace and method of heating materialin an oxy-fuel furnace. Embodiments of the present disclosure increasethe convective contribution of heat transfer in an oxy-fuel heatingprocess, decrease the cycle time for achieving certain temperatures,increase efficiency, or combinations thereof.

Referring to FIGS. 1 through 5, according to an embodiment, an oxy-fuelfurnace 100 includes at least two high-momentum oxy-fuel burners 102 andan enclosure 104 generally defining a combustion zone within theoxy-fuel furnace 100. The enclosure 104 is any suitable geometry and isdefined by a furnace wall boundary 108. For example, in one embodiment,the enclosure 104 is a cuboid or generally cuboid geometry. In anotherembodiment, the enclosure 104 is a cylindrical or generally cylindricalgeometry. The enclosure includes a central region, for example, definedby the burner axis of each of the burners 102, the burner axis being aline extending from the middle of the burner. The oxy-fuel furnace 100includes other suitable features as are necessary to maintaincombustion, heating, other operational conditions, or combinationsthereof.

The enclosure 104 is configured for containing at least a portion of avortex 106 of combustion gases, such as a furnace-scale vortex. Thevortex 106 is formed by offset firing of the burners 102 that entrainssurrounding combustion gases into a flame zone within the enclosure 104,thereby resulting in a churning (or equilibration of gases) that formsthe vortex 106, for example, by transporting the combustion gases. Inone embodiment, the vortex 106 is used with spacious combustion,combustion achieved by entrainment of furnace gases in a flame zone. Inone embodiment, the burners 102 form two different furnace gasrecirculation currents, for example, a horizontal component and avertical component that constrict the vortex 106 due to differentialpressure within the enclosure 104.

The burners 102 are arranged and disposed for forming the vortex 106.The oxy-fuel furnace 100 includes two of the burners 102 (see FIGS. 1and 2), three of the burners 102, four of the burners 102 (see FIG. 3),or more than four of the burners 102. As shown in FIG. 1, in oneembodiment, the burners 102 are positioned on opposite sides of theenclosure 104 on the furnace wall boundary 108, in a staggeredorientation. As shown in FIG. 2, in one embodiment, two of the burners102 are positioned on the furnace wall boundary 108 in an angularconfiguration. As shown in FIG. 3, in one embodiment, four of theburners 102 are positioned on the furnace wall boundary 108 in anangular configuration. Other embodiments include combinations of theseembodiments.

The burners 102 are any suitable burners capable of being used underhigh-momentum conditions, such as, the burners disclosed in U.S. Pat.App. Pub. No. 2007/0254251, which is incorporated by reference in itsentirety, and/or a high shape factor burner. As used herein, the term“high-momentum” refers to flow of gases through at least one channel ofpassageway of the burner 102 that is greater than about 5 lb-ft/s². Insome embodiments, flow of gases through at least one channel ofpassageway of the burner 102 is greater than about 10 lb-ft/s², forexample, as with natural gas having a flow rate between about 10lb-ft/s² and 70 lb-ft/s², enabling firing at higher rates, improvingcycle times, reducing localized overheating (such as overheating ofthermocouples), or combinations thereof. As used herein, the term “highshape factor burner” refers to a burner having a nozzle perimeter ormultiple perimeters that is/are greater than a perimeter of a circularnozzle. For example, a relative perimeter ratio (P_(rel)) is a ratio ofthe perimeter of nozzle(s) of a high shape factor burner (such as, anon-circular burner) in comparison to the perimeter of a circularnozzle. For nozzles having areas of 1.0 in², a circular nozzle has aperimeter of 3.54 inches. Thus, a high shape factor burner having anozzle with a 1.0 in² has a perimeter that is greater than 3.54 inches.In one embodiment, the high shape factor burner has a relative perimeterratio of 1.96.

Referring to FIGS. 4 and 5, in one embodiment, two or more of theburners 102 form the vortex 106 by being angled at angle θ, with respectto the furnace wall boundary 108. The angle θ corresponds with thespecific configuration of the oxy-fuel furnace 100 (for example, thegeometry, the size, or combinations thereof), the materials to be heated(for example, metal or metallic materials, such as, ingots, sheets, castmaterials, forged materials, aluminum, iron, steel, ferrous materials,non-ferrous materials, or combinations thereof), other suitableoperational considerations (for example, flow rates, compositions ofoxy-fuel, enclosure pressure, enclosure material, etc.), or acombination thereof. The material heated is positioned at any suitableportion within the enclosure 104, for example, on the bottom of theenclosure 104. In one embodiment, the oxy-fuel includes a composition ofat least 50 mol % oxygen in the oxidizer (for example, from an oxidizerflow of 95 mol % oxygen) and fuel (for example, natural gas, propane,syngas, low Btu fuels, etc.).

In one embodiment, the angle θ is greater than 15 degrees, greater than30 degrees, greater than 45 degrees, greater than 60 degrees, less than75 degrees, less than 60 degrees, less than 45 degrees, less than 30degrees, or any suitable range sub-range, combination, orsub-combination thereof.

In one embodiment, the oxy-fuel furnace 100 enhances mixing and furnacegas entrainment that reduces the peak flame temperature and thermal NOxgeneration. The enhanced mixing is caused by the burners 102 creating alower pressure region having a first pressure within the vortex 106 anda higher pressure region having a second pressure that is proximal tothe furnace wall boundary 108 of the enclosure 104.

The force (F_(inl)) brought into the enclosure 104 of the furnace 100 bythe flow of combustion gases through the burner 102 can be representedas shown in Equation 1:

F _(inl)=ρ_(inl) ·Q _(inl) ·u _(inl)   (Eq. 1)

As used in Equation 1, ρ_(inl) refers to the density of combustion flowentering the enclosure 104 (for example, as measured in lb/ft³,dependent upon flame temperature). u_(inl) refers to the velocity ofinlet flows entering the enclosure 104 (for example, as measured inft/s). Q_(inl) refers to the total inlet flow rate into the enclosure104 (for example, as measured in ft³/s).

In one embodiment, the entrainment of furnace gases into the flame isenhanced by using nozzles in one or more of the burners 102 that have ahigh shape factor. The actual flow achieved by strong interaction of thenozzles with the furnace gases can be represented as shown in Equation2:

F _(inl)=ρ_(inl)·(Q _(inl) ·P _(rel))·u _(inl)   (Eq. 2)

As used in Equation 2, P_(rel) refers to the relative perimeter rationand Q_(inl)·P_(rel) refers to the total actual inlet flow rate (forexample, as measured in ft³/s).

The vortex is generated by the balance of forces brought into thefurnace and viscous dissipation (F_(visc)) of these flows in thefurnace, given by Equation 3:

F _(visc)=ρ_(furn) ·V _(furn)·(u _(t) ² /d _(e))   (Eq. 3)

As used in Equation 3, ρ_(furn) refers to the density of furnace gaseswithin the enclosure 104 (for example, as measured in lb/ft³, dependentupon flame temperature). V_(furn) refers to the volume of the enclosure104 in the oxy-fuel furnace 100 (for example, as measured in ft³). u_(t)refers to the tangential velocity of the Vortex at diameter d_(e) insidethe enclosure 104 (for example, as measured in ft/s). d_(e) refers to acharacteristic dimension of the Vortex 106 (for example, an equivalentdiameter measured in ft).

In one embodiment, the angular velocity (u_(ω)) of the vortex 106 isdefined using Equation 4, which is based upon consolidation of equations2 and 3:

u _(ω)=√(ρ_(rat)·((Q _(inl) ·u _(inl) ·d _(e))/V _(furn)))/πd _(e)  (Eq. 4)

As used in Equation 4, ρ_(rat) refers to a density ratio of inlet flows(ρ_(inl)) to furnace gases (ρ_(furn)). The density ratio is between 0.8for air-fuel combustion and 0.6 for oxy-fuel combustion due to thedifference in flame temperature.

In one embodiment, the burners 102 enhance a convective heat transfercomponent (in addition to a radiative heat transfer component) toincrease uniformity and/or efficiency of heating. For example, in oneembodiment, a vortex-induced component of the convective heat transfercomponent increases uniformity and efficiency by using the burners 102.The vortex-induced component is achieved by arranging and/or orientingthe burners 102 such that the vortex 106 is formed and maintained withinthe enclosure 104. In one embodiment, the convective heat transferreduces or eliminates direct impact of a flame on the material to beheated.

In one embodiment, the vortex-induced component of the convective heattransfer component impacts between 15% and 75% of the (plan-view) areaof the enclosure 104. In further embodiments, the convective heattransfer component impacts between 30% and 60%, between 30% and 45%,between 45% and 60%, about 15%, about 30%, about 45% about 60% about75%, or any suitable range, sub-range, combination, or sub-combinationthereof. In one embodiment, the vortex-induced component is increased byincreasing the angular velocity (u_(ω)) of the vortex 106.

Referring to FIG. 6, the uniformity and efficiency of the heating withthe burners 102 forming the vortex 106 is improved in comparison toone-sided firing and opposed (but not staggered) burning. FIG. 6 showsprofiles of heating steel ingots with the enclosure 104 being in a pitfurnace under different configurations. A vortex-induced heating profile602 is based upon using the burners 102 to form the vortex 106 asdescribed herein. A one-sided-burner heating profile 604 is based uponusing one-sided firing, positioning the steel ingots at an end distalfrom a burner and moving the steel ingots toward an end proximal to theburner. An opposing-burner heating profile 606 is based upon having twoburners on opposing walls of a furnace. The jets collide and have thetendency to overheat the steel ingots at the center of the pit furnace.The opposing-burner heating profile 606 also creates large heat fluxgradients in comparison to the other configurations.

As shown in FIG. 6, the vortex-induced heating profile 602, includingthe vortex-induced component, is maintained within a temperature rangeof less than 25° F. In further embodiments, the vortex-induced heatprofile 602 including the vortex-induced component is maintained withina temperature range of less than 10° F., less than 5° F., or issubstantially constant. In contrast, the one-sided-burner heatingprofile 604 and the opposing-burner heating profile 606 exceed thetemperature range of 25° F.

Referring to FIG. 7, the surface temperature of material heated underthe vortex-induced heat profile 602 and the opposing-burner heatingprofile 606 described above are shown over time. Each profilecorresponds with a maximum surface face temperature profile 702 and anaverage face temperature profile 704. The maximum surface facetemperature profile 702 is substantially consistent over time for thevortex-induced heat profile 602 and the opposing-burner heating profile606. The average face temperature profile 704 bifurcates over timepermitting a decrease in cycle time for achieving a predeterminedaverage face temperature under the vortex-induced heat profile 602 incomparison to the opposing-burner heating profile 606. In oneembodiment, the decrease in cycle time is at least 10%, between 10% and20%, about 15%, or any suitable range, sub-range, combination, orsub-combination thereof.

FIGS. 8 and 9 comparatively illustrate the heating of the materialwithin the enclosure 104 according to the vortex-induced heat profile602 (see FIG. 9) in contrast to the opposing-burner heating profile 606(see FIG. 8). Specifically, FIG. 8 illustrates the temperature of wallswithin an enclosure heated by the opposing-burner heating profile 606and FIG. 9 illustrates the temperature of the furnace wall boundary 108heated by the vortex-induced heat profile 602. FIG. 8 shows that theopposing-burner heating profile 606 forms a hot spot 802. FIG. 9 showsthat the vortex-induced heat profile 602 forms a more uniformtemperature gradient, for example, having no regions of the furnace wallboundary 108 that exceed the temperature of the furnace wall boundary108 proximal to the burner 102, thereby allowing increased amounts ofenergy input into the oxy-fuel furnace 100 and/or reducing cycle timesfor achieving a predetermined temperature.

In one embodiment, the enclosure 104 has dimensions of 24 ft×9 ft×14 ft.In a heating process achieved in the enclosure 104, the heating processuses an average of about 10 MMBTU/hr of air-fuel firing rate and about 6MMBTU/hr of oxy-fuel firing rate (assuming 45% and 75% available heat inthe enclosure 104, respectively) to form the vortex 106. The angularvelocity (u_(ω)) of the vortex 106 is calculated, for example, basedupon Equations 1 through 4 above, and depends upon the fuel used and theburner used. For example, air fuel combustion with a staggered burnerconfiguration (see FIG. 1) results in an angular velocity (u_(ω)) of0.099 rad/s and an angled burner configuration (see FIG. 2) results inan angular velocity (u_(ω)) of 0.087 rad/s. Low-momentum oxy-fuelcombustion with a staggered burner configuration (see FIG. 1) results inan angular velocity (u_(ω)) of 0.035 rad/s and an angled burnerconfiguration (see FIG. 2) results in an angular velocity (u_(ω)) of0.031 rad/s. High-momentum oxy-fuel combustion with a staggered burnerconfiguration (see FIG. 1) and circular nozzles results in an angularvelocity (u_(ω)) of 0.079 rad/s and an angled burner configuration (seeFIG. 2) results in an angular velocity (u_(ω)) of 0.070 rad/s.High-momentum oxy-fuel combustion with a staggered burner configuration(see FIG. 1) and non-circular nozzles results in an angular velocity(u_(ω)) of 0.111 rad/s and an angled burner configuration (see FIG. 2)results in an angular velocity (u_(ω)) of 0.097 rad/s.

In view of such differences, in one embodiment of the disclosure, theburners 102 of the furnace 100 are arranged and operated such that thevortex has an angular velocity that is greater than a correspondingangular velocity for an air-fuel combustion vortex that would be formedby air-fuel combustion, for example, being at least 0.07 radians persecond. In one embodiment, the vortex 106 formed by combusting theoxy-fuel with the burners 102 having non-circular nozzles has an angularvelocity that is 10% greater than a vortex that would be formed byair-fuel combustion, 40% greater than the vortex 106 formed by theburners 102 having the circular nozzles, 200% greater than a vortex thatwould be formed by low-momentum oxy-fuel combustion, or a combinationthereof.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for heating material in an oxy-fuel furnace, the methodcomprising: combusting oxygen and fuel with an oxy-fuel burnerarrangement in the oxy-fuel furnace forming combustion gases; andmaintaining a vortex including the combustion gases within a centralregion of an enclosure of the oxy-fuel furnace; wherein the oxy-fuelburner arrangement comprises a plurality of high momentum oxy-fuelburners arranged at an angle to generate the vortex, the angle beinggreater than 15 degrees but less than 75 degrees with respect to afurnace wall boundary of the enclosure.
 2. The method of claim 1,wherein at least one of the plurality of high momentum oxy-fuel burnersincludes a high shape factor nozzle.
 3. The method of claim 1, whereinthe plurality of high momentum oxy-fuel burners includes staggeredburners.
 4. The method of claim 1, wherein the plurality of highmomentum oxy-fuel burners includes two burners.
 5. The method of claim1, wherein the plurality of high momentum oxy-fuel burners includes fourburners.
 6. The method of claim 1, wherein the plurality of highmomentum oxy-fuel burners includes more than four burners.
 7. The methodof claim 1, wherein the vortex has an angular velocity that is greaterthan a 0.07 radians per second.
 8. The method of claim 1, wherein theangle is between about 30 degrees and about 60 degrees.
 9. The method ofclaim 1, wherein the vortex induces convective heat in an area of theenclosure, the area being between about 15% and about 75% of theenclosure.
 10. The method of claim 1, wherein the vortex inducesconvective heat in an area of the enclosure, the area being betweenabout 30% and about 60% of the enclosure.
 11. The method of claim 1,wherein the enclosure has a first pressure within the vortex that isless than a second pressure that is proximal to the furnace wallboundary of the enclosure.
 12. The method of claim 1, further comprisingheating metal within the enclosure.
 13. The method of claim 1, furthercomprising heating aluminum within the enclosure.
 14. The method ofclaim 1, wherein the vortex increases convective heating within theenclosure.
 15. The method of claim 1, wherein the vortex increasesuniformity of heating within the enclosure.
 16. A method for heatingmaterial in an oxy-fuel furnace, the method comprising: combustingoxygen and fuel with an oxy-fuel burner arrangement in the oxy-fuelfurnace forming combustion gases; and maintaining a vortex including thecombustion gases within a central region of an enclosure of the oxy-fuelfurnace; wherein the vortex has an angular velocity of greater than 0.07radians per second.
 17. The method of claim 16, wherein at least one ofthe plurality of high momentum oxy-fuel burners includes a non-circularnozzle geometry.
 18. The method of claim 16, wherein the plurality ofhigh momentum oxy-fuel burners includes staggered burners.
 19. Themethod of claim 16, wherein the angle is greater than 15 degrees butless than 75 degrees with respect to a furnace wall boundary of theenclosure.
 20. An oxy-fuel furnace, comprising: an oxy-fuel burnerarrangement including at least two high momentum oxy-fuel burners havinghigh shape factor nozzles; and an enclosure; wherein the oxy-fuel burnerarrangement comprises a plurality of high momentum oxy-fuel burnersarranged at an angle to generate a vortex, the angle being greater than15 degrees but less than 75 degrees with respect to a furnace wallboundary of the enclosure; wherein the vortex increases convectiveheating within the enclosure and uniformity of heating within theenclosure.